Basic subsystems for the architecture of mobile cellular communication systems are a core network and a radio access network.
As shown in FIG. 11, the core network comprises a gateway switching center GMSC, a mobile switching center MSC, further a home location register HLR, and a visitor location register VLR. The gateway switching center GMSC is connected to a plurality of mobile switching centers MSC and each mobile switching center MSC is connected to a visitor location register VLR. Different visitor location registers VLR are connected to the home location register HLR which is also linked to the gateway switching center GMSC.
As also shown in FIG. 11, each mobile switching center MSC serves a plurality of base station controllers BSC being itself connected to a plurality of base transceiver stations BTS for link establishment to mobile stations MS roaming in the area covered by the mobile cellular communication network.
The architecture of a mobile cellular communication network allows interworking between a fixed network such as a public switched telephone network PSTN like a national telephone network, an integrated services digital network ISDN, a public switched data network PSPDN and/or a public land mobile network PLMN like a global system for mobile communications GSM, a digital cellular system DCS 1800, a personal communication system PCS, and/or a third generation 3GPP mobile communication system.
Operatively, the home location register HLR stores two types of information, i.e. subscriber information and mobile information to allow incoming calls to be routed to a mobile station MS. Further, the visitor location register VLR is a functional unit that dynamically stores mobile station information, e.g., location area in case a mobile station is located in an area covered by the visitor location register VLR, and also subscriber profiles for visiting subscribers.
Further, the mobile switching center MSC performs necessary switching functions required for mobile stations MS covered by the mobile service switching center MSC. Also, the mobile switching center MSC monitors the mobility of its mobile stations MS and manages the necessary resources required to handle and update location registration procedures.
Besides the components of the core network of a mobile cellular communication network discussed so far, also components necessary to achieve a radio access to the mobile station must be provided in an access network. The access network comprises a plurality of, e.g., base station subsystems BSS for 2nd generation mobile communication systems or radio network controllers RNC for 3rd generation mobile communication systems. These subsystems correspond to physical equipment providing radio coverage to prescribed geographical areas known as cells illustrated as hexagonal geographical regions in FIG. 11. Each base station subsystem BSS contains equipment required to communicate with the mobile station MS.
Operatively, a base station subsystem BSS provides a control function implemented in a base station controller BSC/a radio network controller RNC and a transmitting function performed by a base station transceiver system BTS. The base station transceiver system BTS corresponds to a radio transmission equipment and covers each cell. A base station subsystem BSS can serve several cells because it may comprise multiple base transceiver systems BTS.
In mobile communication systems of the type outlined above the control of communication processes is achieved through signaling messages between control instances. Signaling messages are transferred both downlink to the mobile station for mobile terminating control signaling and uplink for mobile originating control signaling.
An example for mobile terminating control signaling in a mobile cellular mobile communication networks is the establishment of mobile terminating calls. This mobile terminating control signaling may be separated into two different phases. During establishment of a mobile terminating call the first phase consists of the following steps:                analysis of the called number (B-number) in the gateway switching center GMSC;        interrogation of the home location register HLR;        provision of a roaming number MSRN/TLDN through the visitor location register VLR;        routing of the call from the gateway switching center to the visited VMSC;        sending a paging request to an access node of the access network for further submission by the access node—e.g., a base station controller BSC or a radio network controller RNC—to the mobile terminal, e.g., by broadcasting.        
Further, the second phase of a mobile terminating call establishment comprises the steps:                receiving a paging response from the called mobile station; and        processing of the paging response and completion of the call setup.        
It should be noted that between the mobile switching center MSC and the activated base station controller BSC handling the paging these two phases are treated as completely independent signaling transactions. Therefore, these different signaling transactions must be linked in the mobile switching center MSC. Within existing mobile communication networks this can be done, as the paging request is initiated by the same mobile switching center MSC that also receives the related paging response from the activated base station controllers. In other words, the two phases outlined above may be linked since each base station controller BSC is served by a single mobile switching center MSC only.
However, this simple mechanism will not work in case the 1:n relationship between a mobile switching center MSC and related base station controllers BSC is generalized to a case where each base station controller may be served by a plurality of mobile station centers MSC to increase network capacity and reliability.
In other words, when the base station controller BSC may initiate paging responses to mobile switching centers MSC different from the mobile switching center issuing the related paging request, the link between the two phases of mobile terminating call establishment outlined above cannot be achieved using the existing mechanisms as the mobile switching center MSC receiving a paging response may differ from the mobile switching center MSC initiating a related paging request.
Nevertheless, this might be a pre-requisite to meet future service demands since for large mobile cellular communication networks a static link between access nodes and mobile switching centers MSC could turn out to be an inefficient way of using the core network. When subscribers concentrate in one area—e.g., urban areas during business hours—or during disasters hitting only a certain area of the mobile communication network, the occasion would occur that a lot of calls overload a specific mobile switching center MSC while other mobile switching centers MSC in the mobile cellular communication network still have excess capacity to handle calls.
The problems become even more difficult when mobile communication networks cover very large areas, e.g., spanning more than one time zone. There exists no possibility to do load sharing over available mobile switching centers MSC to spread high load during peak hours in different time zones, e.g., between Dallas and Los Angeles.
Similar problems as outline above with respect to mobile terminal terminating signaling traffic also exist with respect to mobile terminal originating signaling traffic when access nodes have the choice to access different mobile switching centers MSC.
For BSS (TR 23.913 v 1.0.0) there is described a subscriber based mechanism for load distribution for 3rd generation mobile cellular communication systems. Mobile station originating signaling messages are distributed to different mobile switching centers MSC on the basis of the subscriber identity comprised in a signaling message. However, this approach requires the analysis of signaling messages to find subscriber identities and therefore introduces delays as possible bottlenecks.
Referring only to subscriber identities could decrease the scope of flexibility. Further, when a mobile switching center is out of order related subscribers cannot receive signaling messages.